The polluting exhaust gases and residues are in particular nitrogen monoxide and hydrocarbons that have not been burned in the combustion chamber of the engine.
In prior art catalytic converters the catalyst means require high operating temperatures to operate with good efficiency. The temperature of the exhaust gases and residues is totally appropriate to this. However, the trap means, which are immediately downstream of the catalyst means, are not good at withstanding the high temperatures of the gases and residues, especially when "heavily loaded", for example on a freeway. As a result the efficiency of the trap means is less than optimal if the temperature rises too high and tends to fall rapidly because the materials from which they are made deteriorate.
Also, the trap means have optimal efficiency over a relatively narrow range of temperatures that rarely includes the temperature of the exhaust gases and residues.